This invention relates generally to photodetectors and, more particularly, to avalanche photodiodes (APDs).
In a typical back-illuminated APD formed in a Group III-V compound semiconductor body, a light (or radiation) input signal to be detected is directed toward the substrate or bottom side of the body through the opening in an annular contact. This signal, which illustratively has a wavelength in the range of about 1.1-1.7 xcexcm, is absorbed in an InGaAs layer where photocarriers (electron-hole pairs) are generated. Under the influence of an applied voltage the holes drift toward a p-n junction that is located near to and intersects the top surface of the body. In the junction region avalanche multiplication takes place. This multiplication provides for amplification of the input signal. As a result, the receiver using an APD can detect significantly lower optical power signals than a photodetector that does not have an amplification region. The improvement in detection limit is proportional to the amount of multiplication M taking place. The multiplication, in turn, is proportional to the applied voltage. Typically voltages in the range of about 20-100 V are required depending on the desired M and the particular device geometry. The arrival rate of the multiplied carriers (i.e., amplified current), which are collected at a p++-type electrode on the passivated p-side of the junction, is proportional to the intensity of the incident signal and the applied voltage.
The passivation is provided by a relatively thin dielectric (e.g., silicon nitride) layer (or pair of layers). An opening is etched in the nitride layer to expose the underlying, top surface of the p-n junction region, and metal is deposited in the opening so as to form the p++-type electrode and thereby make electrical contact to the p-side of the junction. This electrode, which is typically circular, also serves as a bonding pad.
The nitride layer is typically made as thin as possible consistent with its passivation function. Illustratively, a single nitride layer about 0.2 xcexcm thick is used. Although thicker single layers are possible, they are typically avoided because thicker layers require longer processing time, and single layers thicker than about 0.4 xcexcm tend to crack. To avoid cracking thicker single layers can be fabricated if the passivation process is changed, but known process alternatives can lead to less than adequate passivation properties. Finally, the cracking problem can be circumvented by forming the thick nitride layer as a composite of thinner nitride layers, but once again such a process would increase cost without any apparent performance benefit.
We have found that this type of APD has poor reliability in a non-hermetic (i.e., moisture-containing) environment. Yet, there is a need in the optical components art, particularly those intended for low cost applications, for an APD that can operate reliably in non-hermetic environments. Because the packaging of such an APD would not require a hermetically sealed enclosure, it would allow for easier access to the APD and thus would facilitate optical alignment of the APD to other devices (e.g., optical fibers), thereby reducing cost of assembly. Cost would be further reduced and yields improved because the APD package would not have to meet or be tested for strict hermeticity requirements (e.g., a typical hermetic enclosure must have a package leak rate of less than about 10xe2x88x928 atm/cmxe2x88x922-sec.).
We have discovered the failure mechanism responsible for the poor reliability of such APDs in non-hermetic environments. More specifically, the relatively high voltages applied to the APD in combination with the presence of a moisture-containing ambient (i.e., water vapor, humidity) lead to significant electric fields across the silicon nitride passivation layer, especially where the p-n junction intersects the top surface of the semiconductor body. The relatively high electric field across the nitride layer causes leakage current to flow through it. In the presence of a moisture-containing ambient and sufficient current, the nitride layer oxidizes. Eventually the nitride layer is breached by the oxidation process, thereby exposing portions of the top surface of the underlying semiconductor body to the moisture-containing ambient and to the relatively high electric fields. Once exposed, the semiconductor rapidly oxidizes and the device fails. In addition, we have found that the presence of electrostrictive forces can increase the susceptibility of the device to degradation in moist environments even at relatively low RH depending on the position of the wire bond relative to the p-side contact and the nitride surface.
In accordance with one aspect of our invention, this reliability problem is addressed in a non-hermetic APD that comprises an InP/InGaAsP-containing Group III-V compound semiconductor body and a p-n junction formed in the body. Typically the junction intersects a top surface of the body. A patterned dielectric layer is formed on the surface so as to cover at least those regions of the surface that are intersected by the junction. An electrode is formed in an opening in the dielectric layer so as to make electrical contact with one side of the junction. Importantly, the thickness of the dielectric layer is sufficient to reduce the leakage current through it to less than about 1 nA when the operating voltage is in the range of about 20-100 V. In accordance with a preferred embodiment, the thickness of the dielectric layer is greater than about 2 xcexcm when the applied voltage is in excess of about 20 V. Moreover, the composition of the dielectric layer may be either inorganic (e.g., a silicon nitride) or a combination of inorganic and organic materials.